1. Field of the Invention
The present invention relates to a method of forming an avalanche semiconductor photodetector device of the type with separate absorption and multiplication zones.
2. Description of the Prior Art
For increasing the performances of avalanche photodetectors, (reduction of the noise) heterostructures are conventionally used.
The absorption takes place mainly in the material with narrow prohibited band width and multiplication in material with wide prohibited band width. In such a device, carriers, for example holes, may be stored temporarily by the potential barrier of the heterojunction, leading to response times which are too high, typically of a few tens of nanoseconds for a device with a junction of Indium-Gallium-Arsenic/Indium-Phosphorous (InGaAs/InP). structure.
In a second approach of the prior art, this problem may be solved by replacing the heterojunction by a region of variable prohibited band width. Easy to realize in the case of the junction system of the Gallium-Arsenic/Aluminium-Gallium-Arsenic (GaAs/AlGaAs) type, such a variation is technically very difficult to accomplish without introducing mesh detuning in a system such as the junctions of the Indium-Gallium-Arsenic/Indium-Phosphorous (InGaAs/InP) type. That is due among other things to the difficulties in mastering the exact composition of the intermediate quaternary compound.
In a third approach of the prior art, the response time may also be reduced by replacing the abrupt heterojunction by a succession of thin layers of the two end compounds, Indium-Phosphorous and Indium-Gallium-Arsenic for example.
Such a method was described in the article by CAPASSO et al.: "Pseudo-quaternary GaInAsP semiconductors: A new Ga.sub.0.47 In.sub.0.53 As/InP graded gap superlattice and its applications to avalanche photodiodes" published in the US review: "Applied Physics Letters", vol. 46, No. 11, 1st Dec. 1984, pages 1193-1195.
The result sought is to produce a "pseudo-quaternary alloy" which has the same mean local potential characteristics as an alloy of variable composition in the second approach of the prior art. A constant mesh length is imposed on the system. The mesh is formed by a well and the potential barrier which follows it. That leads to considering systems with very numerous wells, typically twenty or so.
This method therefore does not allow optimized avalanche photodetectors to be obtained. The high number of semiconductor "slices" to be formed leads to manufacturing difficulties.
The purpose of the invention is to provide a method of forming avalanche semiconductor photodetector devices palliating the disadvantages of the prior art.
The structure of the invention comprises, as in the case of the third approach of the prior art, a lattice of layers of alternate semiconductor materials.
However, the starting approach is different, which allows, for a comparable operating speed characteristic, the number of alternate layers to be appreciably reduced, that is to say the structure of these layers and therefore the structure of the detector to be optimized.
In addition to the manufacturing economies obtained, by reducing the number of interfaces, the quality of the device is improved since each interface generates defects.